A conventional method for manufacturing an optical fiber 100 as shown in FIG. 4 will be described below. FIG. 4A is a view showing the cross section of the conventional optical fiber 100 and FIG. 4B is a view showing a refractive index distribution of the optical fiber 100.
First of all, a ring portion 102 is formed by depositing glass particles on the inner face of a starting pipe 101 as a starting material by the MCVD method (Modified Chemical vapor Deposition Method, Internal CVD method). Then, a glass rod 105 having a core portion 103 and a depressed portion 104 is inserted into the ring portion 102 and integrated by rod-in collapse to fabricate a glass body 106. After a jacket portion 107 is sooted outside the glass body 106, a preform 100a is produced by vitrification. Thereafter, the preform 100a is heated and drawn to fabricate the optical fiber 100.
In the conventional manufacturing method as described above, the ring portion 102 is formed by the MCVD method, using the starting pipe 101 having a thickness 101a from 2 mm to 3 mm. In practicing the MCVD method, the starting pipe 101 is heated using an oxyhydrogen flame. Further, in the rod-in collapse and the process of sooting the jacket portion 107, the starting pipe 101 is also heated using an oxyhydrogen flame. Therefore, hydroxyl group contained in the oxyhydrogen flame enters the surface of the starting pipe 101. This hydroxyl group may intrude near the core portion 103, and when the preform 100a is the optical fiber 100, light is absorbed into the hydroxyl group, giving rise to a situation where the transmission loss is increased.